Body implantable electrical leads form the electrical connection between an implantable medical device (IMD), such as a cardiac pacemaker and/or implantable cardioverter-defibrillator (ICD), and body tissue, such as that of the heart, which is to be electrically stimulated. As is well known, the leads connecting IMDs with the heart may be used for pacing or for sensing electrical signals produced by the heart, or for both pacing and sensing in which case a single lead serves as a bidirectional pulse transmission link between the pacemaker and the heart. An endocardial type lead, that is, a lead that is inserted into a vein and guided therethrough into a cavity of the heart, comprises a lead body that carries along its distal end portion one or more electrodes designed to contact the endocardium, the tissue lining the inside of the heart. The lead body further includes a proximal end carrying an electrical connector assembly adapted to be received by the IMD. By way of example, the connector assembly may be of the trifurcated type comprising first and second connector branches that may conform to the DF-1 standard for supplying electrical impulses to shocking electrodes, and a third connector branch that may conform to the IS-1 standard for connecting the IMD to a tip electrode and a ring electrode. Alternatively, the connector assembly may simply comprise a single in-line or coaxial connector of the IS-1 type or conforming to the proposed quadripole IS-4 or DF-4 standard. The IS-4 and DF-4 connector assemblies are less bulky than trifurcated assemblies and comprise a coaxially arranged pin terminal contact and three ring terminal contacts. A flexible cable or coil conductor surrounded by an insulating sheath or housing of a polymer such as silicone rubber or polyurethane electrically connects each terminal contact on the electrical connector assembly with an associated electrode on the distal end portion of the lead.
The generation of static electricity caused by rubbing two substances together is called the triboelectric effect. The prime sources of static electricity are insulators typically made of synthetic materials. Voltage levels generated by these insulating sources can be extremely high since their charges are not readily dissipated or distributed over their surfaces or conducted to other objects. The accumulation of electrostatic charge in the form of rubbing, friction-induced triboelectric charge has been a persistent problem with endocardial leads. Such charges may accumulate, for example, as a result of the electrical conductor coils rubbing against the surfaces of the insulating housing of the lead body such as the walls of the conductor—containing lumen(s) inside the housing.
Static charge build-ups have also been observed on the in-line, quadripole connector assemblies conforming to the proposed DF-4 standard. These connector assemblies typically include a pair of high voltage ring terminal contacts that supply dual shock electrodes. Static charges tend to build up on the insulation between and adjacent to the ring terminal contacts on the connector assembly as a result of the high voltage spikes during defibrillation. The electrical insulation may slowly degrade (similar to being electrically burned) due to the voltage build-up. Similarly, static charges tend to build up on the surfaces of the internal insulating seals between the contact elements within the connector assembly-receiving cavity in the IMD and to progressively burn away these seals.
The dissipation of electrostatic charges is a particular problem in leads carrying cardiomechanical sensors (CMES) such as pressure transducers or accelerometers that generate very low amplitude output signals that are easily lost in the noise produced by the static charges.
Antistatic coatings applied to the charge-accumulating surfaces of lead body housings and connector assemblies have been used to dissipate static charge or to prevent their buildup. These coatings typically comprise polymers filled with low aspect ratio graphite particles, carbon fibers, carbon black, or the like, that render the coatings sufficiently electrically conductive so as to dissipate charge. Such charge dissipation can occur, however, only if the loading levels of the filler materials are high enough to produce percolation, that is, extended, connected networks providing sufficient electrical conductivity. Thus, these coatings tend to be relatively thick and therefore unacceptable for use with leads having small diameters, and with connector assemblies such as the aforementioned DF-4 connector assembly that is small and has tight design tolerances.